As aforesaid, the present invention particularly relates to the manufacture of a tissue product and, thus, to the production and processing of a tissue ply or tissue web, respectively. Tissue is defined as a soft absorbent paper having a low basis weight. One generally selects a basis weight of 8 to 30 g/m2, especially 10 to 25 g/m2 per ply. However, the basis weight may also be up to 50 g/m2 and typically up to 40 g/m2. The total basis weight of multiple ply tissue products resides in a range of 16 up to 100 g/m2 and particularly up to approximately 80, 70 or 50 g/m2. Its density is typically below 0.60 g/cm3, particularly below 0.30 g/cm3 and more preferably between 0.08 and 0.20 g/cm3.
The production of tissue is distinguished from paper production as such by its extremely low basis weight and its much higher tensile energy absorption index (see DIN EN 12625-4 and DIN EN 12625-5). Paper and tissue also differ in general with regard to the modulus of elasticity that characterizes the stress-strain properties of these planar products as a material parameter.
Tissue is made out of pulp, the process and the apparatus, respectively essentially comprising:
I: A forming section including the head box and the forming fabric portion,
II: Pressing or TAD (through air drying)
III: Yankee cylinder or other drying means.
IV: Creping.
V: Other after treatments, monitoring and winding area.
Step I-II in the above-mentioned process steps comprises dryness values of up to 40%. In a TAD for example the ingoing dryness values could approximately be about 20% and the outgoing dryness values could for example be from about 60-65%, which would require further drying, to about 90-95%. Step III and IV are optional.
In the prior art, several techniques are known to impart the tissue ply with a 3-dimensional pattern.
One technique is wet forming of the ply. The definition of wet-forming is that a 3-dimensional pattern is formed in a wet stage while the fibres may still be moved, preferably the fibers are dried in this position. Up to dryness values of approximately up to 40% the fibers are considered to have the ability to move within the web. The wet forming of the ply can be conducted by means of the forming fabric, the TAD fabric or the transfer fabric (i.e. the fabric used to transfer the tissue ply from the forming section to e.g. the TAD section), the ply being in contact with these fabrics so that a pattern is formed, which is imparted by the fabric. This pattern may result to a certain increased volume of the paper and further a textile appearance. Beside the fabrics, such a pattern may be provided in the wet-forming portion by means of rollers, felts or belts. In this context a belt is an alternative of a fabric and comprises a supporting substrate with an applied resin structure or pattern. Similar as in a fabric, also the structure of the belt includes knuckles and pockets, however, not being formed by warp and weft yarns, but by the applied resin structure.
Another technique used in the prior art is embossing. Micro as well as macro embossing and also decorative embossing is known in the prior art for converting tissue paper web obtained from the paper machine to a finished tissue product, wherein converting is also known as processing of the tissue paper web in order to provide one or more tissue plies forming the final tissue paper product. In this understanding the continuous paper web may be named a tissue ply when processed and converted in a converting machine. In addition, a so-called pre-embossing of a tissue ply is known upstream of lamination to a finished e.g. multiply tissue product. The aforementioned processes are done when the paper is dry.
A wet-formed pattern being imparted in a wet state much better maintains its structure at rewetting compared to a pattern introduced in a dried state, e.g. by means of embossing.
In addition, two-ply tissue products are known in the prior art, which products comprise two plies, both plies being produced on e.g. a TAD paper machine, i.e. TAD is used for drying. Each ply on both of its surfaces has a wet-formed pattern (imparted by the TAD fabric). There is a pattern on the fabric side of the ply, i.e. the side of the ply being in direct contact with TAD-fabric, which has the negative surface topography of the TAD-fabric or the imprint of the TAD-fabric. In addition, there is also a pattern on the side of the ply opposite the fabric side, being also imparted by the TAD-fabric, however, being substantially the positive of the TAD-fabric. The two plies are brought together so that the two opposed external surfaces of the two-ply tissue product have the same or opposite appearance. I.e. either the fabric sides represent the external surfaces or the sides opposite to the fabric sides.
A TAD paper is more expensive compared to tissue paper being manufactured in a machine having no TAD section. However, the textile appearance of the more expensive TAD paper and more essentially the higher absorption capacity is more desired by the customer. In contrast there are advantages of tissue paper having no intentionally imparted wet-formed pattern (non-wet-formed tissue paper) compared to TAD paper, such as printability. This also applies vice versa as far as other properties of the respective materials are concerned.